Current hypotheses concerning the underlying biology of tumor formation and progression are that 1) stem and progenitor cells are targets of transforming mutations and, 2) cancer stem cells exist, which have unlimited proliferative potential. Normal stem cells and cancer stem cells both have the ability to self-renew and are thought to utilize similar pathways for self-renewal, proliferation and fate decisions. Thus, the ability to identify stem and progenitor cells in the breast is critical for future investigations of how transforming events in these cells lead to specific types of breast cancers and for understanding the pathways that regulate proliferation of these cells. The identification of stem and progenitor cells in human breast tissue and breast cancers is an active area of current investigation. However, while multiple characteristics of stem/progenitor cells have been described in human breast and mouse mammary tissue, there are no clear cellular markers available to identify specific stem and early progenitor populations in mammary/breast tissue. Nestin is an intermediate filament, which was originally identified as a marker for neuroepithelial stem cells. Recently, nestin expression was identified in the stem/progenitor cells of multiple tissues and has been proposed as a general marker of stem and progenitor cells. We now have preliminary evidence that nestin identifies a small population'of cells in the mouse mammary gland, in a pattern consistent with its expression in stem and progenitor populations. Moreover, we have found that nestin-positive cells are expanded in specific types of Wnt-induced mammary tumors. The goal of this application is to test the hypothesis that nestin expression identifies a population of self-renewing stem/progenitor cells in the normal mammary gland that are expanded in certain mammary tumors. This research will elucidate a new marker for a subset of breast epithelial cells which have the potential to give rise to specific cell types during normal breast development and to specific types of breast tumors. This novel marker may identify a population of undifferentiated breast tumor stem or progenitor cells. [unreadable] [unreadable] [unreadable]